1. Field of the Invention
The present invention relates to electroluminescent devices of compound semiconductor, and more particulary to improvements in electroluminescent devices comprising a Group II - VI compound semiconductor such as zinc sulfide (ZnS) or zinc selenide (ZnSe).
2. Description of the Prior Art
Group II - VI compound semiconductors such as ZnS and ZnSe are generally used as materials for devices, such as blue light-emitting diodes, for producing light with high efficiency over the region of ultraviolet rays to visible rays.
FIG. 14 shows an example of structure conventionally used for electroluminescent devices of such a Group II - VI compound semiconductor. Indicated at 71 is a low-resistance n-type ZnS single-crystal substrate prepared from a ZnS bulk single crystal grown by the halogen chemical transport process, by heat-treating the crystal in molten zinc at 1000.degree. C. for 100 hours. A light-emitting layer 74 of n-type ZnS and an insulating layer 75 of insulating ZnS are successively formed epitaxially over the substrate 71 by molecular beam epitaxy (MBE) or organometallic chemical vapor deposition (MOCVD). Gold (Au) is deposited on the insulating layer 75 by vacuum evaporation to form a positive electrode 77. An ohmic electrode of indium (In) serving as a negative electrode 78 is formed on the rear surface of the low-resistance n-type substrate 71. Thus, a MIS (metal insulator semiconductor) electroluminescent device is fabricated.
Also known is an electroluminescent device wherein a conductive layer is provided between a substrate and a light-emitting layer.
With reference to FIG. 15, the known device comprises a low-resistance n-type ZnS substrate 83 having a resistivity of 10 to 1 ohm-cm and a thickness of 300 to 1000 .mu.m, a low-resistance n-type ZnS conductive layer 84 having a resistivity of 10.sup.-2 to 10.sup.-3 ohm-cm and epitaxially formed over the substrate 83, for example, by MBE from ZnS with Al, Cl or the like added thereto, a low-resistance n-type ZnS light-emitting layer 85 and a ZnS high-resistance layer 86 which are formed successively over the layer 84, for example, by MBE, an ohmic electrode 82 formed on the rear surface of the substrate 83 by depositing In thereon by vacuum evaporation and heat-treating the deposit in a high-purity gas atmosphere at 450.degree. C. for several seconds to several minutes, an electrode 87 formed on the high-resistance ZnS layer 86 by depositing Au thereon by vacuum evaporation, and lead wires 81 and 88 suitably arranged.
With these conventional electroluminescent devices, the current injected via the electrodes 77, 78 or 82, 88 flows through the device over a wide region, so that the current density in the light-emitting layer 74 or 85 is small. This makes it difficult to obtain luminescence with high brightness. Furthermore, the light produced by the emitting layer 74 or 85 radiates through the device in every direction. It is therefore likely that the light produced will not be taken out of the device efficiently.
Either one of the foregoing constructions of electroluminescent devices may be used for fabricating a monolithic display device which comprises a multiplicity of minute luminescent chips having a unit size of 100 .mu.m and prepared by forming a minute discrete pattern on the substrate. When current is passed through the device, a majority of current loss occurs in the substrate. It is therefore likely that the device is high in the series resistance of the chips and has an impaired insulating property between the chips.
An object of the present invention, which has been accomplished in view of the foregoing situation, is to provide an electroluminescent device of Group II - VI compound semiconductor adapted to produce light with high brightness.